The present invention relates to a light transmitting system using a graded index optical fiber comprising a core in which dielectric constant continuously decreases from its center along its radius and a cladding which has a uniform dielectric constant larger than that of the core at its interface with the cladding.
As information to be transmitted increases, a light transmitting system, which can transmit through an optical fiber a large amount of information with low loss of energy, has become more and more attractive so that various optical fibers have been developed and improved in various points and some optical fibers have already become available for practical applications.
The optical fiber which was developed first is a so-called step index optical fiber comprising a core 1 having a uniform dielectric constant .epsilon..sub.1 therein and a cladding 2 having a uniform and lower dielectric constant .epsilon..sub.2 as shown in FIG. 1. The entire distribution of dielectric constant is step-shaped as shown in FIG. 2. In this FIG. 2, the abscissa shows distances from the center of the core 1 in a direction of radius and the ordinate shows dielectric constant. This step index optical fiber, however, cannot have a wide bandwidth because the so-called dispersion is very large. In order to avoid such a defect, the development is made for a so-called graded index optical fiber comprising a core in which a dielectric constant decreases gradually from its center along its radius as shown in FIG. 3. It is known that the transmission characteristics of such a graded index optical fiber are superior to those of a step index optical fiber. A large factor which determines the bandwidth available for transmission through an optical fiber is the distortion of signal caused by group-delay differences among many modes on account of various dispersions. Such dispersion involve material dispersion, waveguide dispersion and multimode dispersion, in which the last-mentioned multimode dispersion affects seriously to a light transmission system through a multimode fiber. To investigate a method for lessening this multimode dispersion, it is necessary to analyze guided modes propagated through an optical fiber. However, the analysis of guided modes through a graded index fiber is so complicated that any rigorous analytical solution cannot be obtained. Therefore, we must be satisfied with a good approximate solution which can be obtained by a numerical and/or theoretical analysis. For example, one of such a theoretical analysis is described in "Journal of the Optical Society of America", Vol. 67, No. 1, pp. 96 to 103.
From such studies a proposal has been made for a graded index fiber the core of which has a parabolic distribution of dielectric constant along its radius and the cladding of which has a uniform distribution thereof, with an arbitrary dielectric constant "step" or "valley" at the core-cladding boundary as shown in FIG. 4. It has been proved that such a graded index fiber is advantageous in the above described multimode dispersion. In this specification such a fiber is called a graded index optical fiber with a valley at the core-cladding boundary. A numerical analysis of guided modes through such a fiber is described in "IEEE Transactions on Microwave Theory and Techniques", (Vol. 1, MTT-24, No. 7, pp. 416 to 421.) There are shown four profiles of distribution of dielectric constant in FIGS. 3(a), (b), (c) and (d) shown in page 419 of this article. In FIGS. 4(a), (b), (c) and (d) of the article there are shown frequency-group delay characteristics with a parameter of guided modes. FIG. 5 of the accompanying drawings is a frequency-group delay characteristic for a graded index optical fiber with a valley at the core-cladding boundary shown in FIG. 4(d) of the above article. It will be seen from this FIG. 5 that there is only a small mode-delay difference in lower order modes such as of zero-th, first and second order as well as in higher order modes. However, the theoretical results by the present inventors have revealed that there are, in fact, a large mode-delay difference among the modes with lower azimuthal number. The result of FIG. 5 may have been probably obtained from an insufficient approximation in the numerical analysis. Thus, a sufficiently wide bandwidth cannot be obtained even through a graded index optical fiber with a valley at the core-cladding boundary, because such a fiber has a large group-delay difference among many modes, especially among modes with lower azimuthal numbers described above.